Add SSE2 version of TransformOne and TransformTwo

[brianpopow]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

This PR adds SSE2 versions of TransformOne and TransformTwo, which are used during webp lossy decoding.

Related to #1786

Profiling results:

master:

PR

Test image was Calliphora.png converted to lossy webp.

note: The Scalar version of TransformTwo calls two times TransformOne, hence the difference in call counts from PR to master

[codecov]

Codecov Report

Merging #1862 (da843f7) into master (0517fec) will decrease coverage by 0%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #1862    +/-   ##
=======================================
- Coverage      87%     87%    -1%     
=======================================
  Files         935     935            
  Lines       48786   48935   +149     
  Branches     6083    6085     +2     
=======================================
+ Hits        42722   42803    +81     
- Misses       5056    5122    +66     
- Partials     1008    1010     +2     

Flag	Coverage Δ
unittests	87% <100%> (-1%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/ImageSharp/Formats/Webp/Lossy/LossyUtils.cs	100% <100%> (ø)
src/ImageSharp/Formats/Webp/Lossy/QuantEnc.cs	96% <100%> (ø)
src/ImageSharp/Formats/Webp/Lossy/Vp8Encoding.cs	100% <100%> (ø)
...ageSharp/Formats/Webp/Lossless/PredictorEncoder.cs	91% <0%> (-7%)	⬇️
.../ImageSharp/Formats/Webp/Lossless/LosslessUtils.cs	92% <0%> (-6%)	⬇️

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[JimBobSquarePants]

Nice speedup. LGTM 👍